1. Field of the Invention
The present invention relates to an improved digital circuit for controlling movement of the needle of a meter by supplying a PWM (Pulse Width Modulation) signal to a coil (load), for example.
2. Description of the Related Art
FIG. 1 shows a conventional PWM signal generating circuit. In FIG. 1, reference numerals 1, 2, and 3 respectively denote an N-bit binary counter, a comparing circuit, and an N-bit latch.
In the above PWM signal generating circuit, N-bit binary data which varies cyclically is output from the N-bit binary counter 1 to the comparing circuit 2. N-bit digital data is input to the N-bit latch 3 and is output therefrom to the comparing circuit 2. The N-bit binary data and the N-bit digital data are compared with one another by the comparing circuit 2, as a result of which a PWM signal is output therefrom.
In the case of driving the needle of a meter by supplying a PWM signal to a coil, that coil generates an audible sound when the frequency of the PWM signal is less than approximately 20 [kHz], and hence there is a demand for a PWM signal generating circuit which is capable of generating a PWM signal having a frequency of approximately 20 [kHz] or higher.
Let it be assumed that 8-bit digital data, for example, is input to the latch 3 in the conventional PWM generating circuit of FIG. 1. When the frequency of the PWM signal is set at approximately 20 [kHz], the minimum pulse width of the PWM signal is logically about 0.195 [.mu.s]. However, it is actually impossible to operate the PWM signal generating circuit at a speed as high enough to generate a PWM signal having a pulse width of approximately 0.195 [.mu.s], resulting in the aforementioned problem that the coil generates an audible sound.
Let it be assumed that 6-bit digital data, for example, is input to the latch 3 in the conventional PWM signal generating circuit of FIG. 1. When the frequency of the PWM signal is set at approximately 20 [kHz], the minimum pulse width of the PWM signal is logically about 0.781 [.mu.s], which is four times greater than in the case of 8-bit digital data. However, use of digital data, comprised of a relatively small number of bits, e.g., 6 bits, incurs the problem that the resolution of the PWM signal output from the PWM signal generating circuit is low.